Tyler'sTwin said:Saltin said something else I found interesting. He thinks altitude training in the off-season has no long lasting physiological benefits after returning to sea level. It's only meaningful in the weeks prior to a targeted event.
If the target race is at sea level, altitude training has proven useful only for some. However, if the race is at altitude, altitude training just prior to the race is deemed beneficial / necessary - and earlier stays at altitude that training season will help the body adapt more quickly to a repeat stay in altitude.
Most of the adaptations relate to physiology other than the often-discussed increase in red blood cell mass. Any meaningful increase in Hkr or Hb typically required staying a minimum of 3 weeks at altitude. A 7-10 day stay will help the body go through the adaptation of plasma volume (which goes down in the early days in altitude, and correct back up after 4-7 days), breathing frequency and basic blood oxygen saturation. Extra red blood cells would be highly useful for performance, but difficult to obtain as a practical matter without resorting to doping.